It is possible to regulate the loads acting on the blades of a wind turbine by modifying the airfoil shape of the blade, particularly at the trailing edge section thereof. EP2153059 discloses a wind turbine blade “having a suction side and a pressure side, which sides are connected at a leading edge and a trailing edge, wherein one or more shape modifiable airfoil sections are defined in the area of the trailing edge of the blade, and wherein said one or more shape modifiable airfoil sections are attached to a blade body, each of the one or more modifiable airfoil sections having an upper skin and a lower skin, a first one of the upper and lower skin being secured to the blade body, and a second one of the upper and lower skin being slidably movable with respect to the blade body, so that a force applied to one of said skins cause said second skin to slide with respect to the blade body, so as to thereby modify the airfoil shape of the trailing edge.”
Such a shape modifiable trailing edge section increases the lift of the blade, but a lift increase can lead to boundary layer separation (or, in general, flow separation) from the aerodynamic surface of the blade (airfoil), which in turn decreases the lift, thus reducing the initial effect.
WO2011026495 discloses a wind turbine blade comprising: “a flap for modifying the aerodynamic surface of the rotor blade; a chamber disposed in the rotor blade and configured such that movement of the flap causes a property change in the chamber, the property change being a change of air pressure within the chamber and/or a change of the volume of air in the chamber; a nozzle disposed on the aerodynamic surface of the rotor blade and in fluid communication with the chamber; wherein the chamber and the first nozzle are configured such that the property change in the chamber generates an air mass flow through the nozzle.”
In the embodiment of FIG. 3 the flap 15 is formed from an elastomeric material and is “a solid member attached to the blade body 17 which is caused to deflect by an actuator ( . . . ) Embedded in the flap 15 are two reservoirs 30 and 31. Upper reservoir 30 is situated near the suction surface 22 and lower reservoir 31 is situated near the pressure surface 23. When the flap 15 deflects, the shape of the reservoirs 30 and 31 also change since they are formed as chambers in the elastomeric material; this in turn causes the volume of each reservoir 30 and 31 to change such that they will either expel air or suck in air. Upper reservoir 30 is attached to a nozzle 34 via means of a conduit 33 and the nozzle 34 is located on the pressure surface 23. Lower reservoir 31 is attached to a nozzle 36 via means of a conduit 35 and the nozzle 36 is located on the suction surface 22. When the flap 15 deflects downwards towards the pressure side 23, lower reservoir 31 is compressed forcing air to be expelled through nozzle 36 which will help the boundary layer to stay attached and prevent or delay separation of the boundary layer. At the same time, upper reservoir 30 will expand which will suck air through nozzle 34 and into the upper reservoir 30, and this suction on the pressure surface will help the boundary layer to stay attached to the airfoil profile.”
But said nozzles on the aerodynamic surface of the blade can create unwanted boundary layer perturbations too.